I often dive in cold dark waters Once or twice.After reading a few reports of survival at sea, I tried a few strobes and settled on the ACR Electronics Firefly Plus. While not a dive specific strobe, people seem to agree that this is one of the top choices for diving.At $36 or so, it costs less than half of the alternatives and holds up well against the competition. I always have one on me whether it is to mark the upline or as a safety should I get lost at sea.